Phase modulation of binary digital data is a basic modulation technique employed in medium-speed data transmission systems employing voiceband transmission channels. Differentially coherent phase-shift-keyed (PSK) modulation, in particular, is widely utilized because no reference carrier wave need be transmitted, thus conserving bandwidth and minimizing error propagation. Data are coded by means of changes in phase rather than by absolute phases. When two discrete phases are selected for encoding, e.g., 0.degree. and 180.degree., binary data are transmitted bit by bit. With four equally spaced discrete phases, e.g., 0.degree., .+-.90.degree. and 180.degree., two bits of data are transmitted in each baud interval. With eight discrete phases, e.g., the integral multiples of 45.degree., three bits of data can similarly be encoded on each allowed phase. Increases in numbers of bits per baud interval are eventually limited by noise characteristics of the transmission channel.
Known techniques for differential PSK modulation of digital data have included making available continuously all the allowable phases in analog form and keying the correct phase to the line as required by the phase encoding; counting down from a high-speed binary counter and controlling the advance or retardation in the frequency-division ratio in accordance with the phase encoding; and exciting ringing oscillations alternately at predetermined phases of a common frequency in accordance with the data encoding and interleaving the ringing oscillations under the control of a sinusoidal envelope to suppress transients arising at phase transitions.
It is an object of this invention to simplify and improve the countdown technique for differential phase-shift-keyed modulation of binary data signals.
It is a further object of this invention to embody a modulator for differential PSK data in relatively simple digital network configurations.